The present invention relates to electronic modules including an aluminum-based metal substrate carrying at least one power component such as a transistor. Such a module is particularly, although not exclusively, applicable to the automotive industry where electronic circuits are used to power engine elements and are placed in a high temperature environment, which makes it more difficult to remove the heat dissipated by the component(s).
A known assembly technique consists in mounting the transistor (or any other heat-dissipated element) on the relative thin aluminum substrate (a few millimeters thick) via a thin insulating layer. That solution area does not exceed the area of the component, presents a large amount of thermal resistance. The thermal inertia of the aluminum substrate is relatively small and does not make it possible to absorb fast variation in the power dissipated by the component without significant temperature variation.
It is an object of the invention to provide a module that satisfies practical requirements better than previously known modules. It is a more specific object to decrease the thermal resistance of the heat-evacuating circuit and to accommodate fast peaks of dissipated heat power.
To this end, the invention provides in particular a module comprising an aluminum-based metal substrate; at least one bar of copper-based material constituting an equipotential connection and fixed to the substrate via an insulating layer; at least one power component in direct contact with the bar via a surface area that is smaller than that of the insulating layer; and a printed circuit card mounted on the bar(s), projecting therefrom, and carrying those components of the module which have a low heat-dissipation .
In addition to the bar(s) carrying at least one power component, the module will typically include one or more bars for carrying power currents and having the outlets of the power component(s) connected thereto.
The bars can be pressed strongly against the substrate by rivets or screws bearing against the card, which in turn bears against the bars, the ends of the rivets or screws being engaged in the substrate. If the screws or rivets are not of electrically-insulating material, they should be separated from the bar(s) by insulating sleeves.
In general, the printed circuit card carries conductive tracks on both of its sides, thus making it possible to provide connections with the bar(s) and enabling components to be mounted on its side facing the substrate where the card extends beyond the bars.
The card can have openings for passing projections of the bar carrying a power component or a coupling for such a component, thereby facilitating assembly.
The invention also provides a method of manufacturing a module of the above-defined kind, comprising the following steps:
soldering one or more power components on at least one bar of copper-based metal;
coating a larger face of the bar remote from the component(s) in a layer of heat-setting insulating material;
mounting a printed circuit card fitted with components that dissipate little heat compared with that dissipated by the power components on the bars;
clamping the resulting assembly against an aluminum-based metal substrate by means of screws or rivets; and
making junctions between the outlets of the power components and corresponding bars.
In practice, it is generally necessary to use bars having a minimum cross-section of about 10 square millimeters (mm2). The insulating material coating the larger faces of the bars can be of a type which, on first heating, flows to fill the pores in the faces between which it is located.